(1) Field of the invention:
The present invention relates to a system and method for controlling an engine output to suppress wheel slippage in an automotive vehicle.
(2) Background art
Japanese Patent Application First Publication (Unexamined) No. Showa 62-45944 published on Feb. 27, 1987 exemplifies a previously proposed system for controlling engine output to suppress drive wheel slippage.
In the previously proposed system, another (second) throttle valve which is externally controllable is installed within a throttle chamber of an intake air passage of a vehicular engine in series with the usual (first) throttle valve which is linked to the accelerator pedal.
During occurrence of slippage affecting drive wheels, the second throttle valve is controlled in three modes, i.e., fully open, hold, and fully closed, according to situations (e.g., magnitude) in which drive wheel slippage occurs. Then, a control system for the second throttle valve, independent of the acceleration system including the first throttle valve, controls an opening angle of the second throttle valve to reduce engine output (torque) so as to suppress drive wheel slippage.
In such a previously proposed system for controlling the engine output as described above, in a case where after start of the engine output reduction control through the second throttle valve at a time during which the opening angles of both first and second throttle valves coincide with each other has passed a predetermined duration of time, engine output control to reduce the engine output is ended. In addition, when the engine output (reduction) control end condition is satisfied, the second throttle valve returns to the fully open position to be set at a high speed which is the same as that carried out during the engine output reduction control to suppress slips on the drive wheels.
Therefore, in a case where engine output reduction control is resumed a very short time after the end of a previous engine output reduction control, a longer interval of time (T.sub.L in FIG. 6) is required for the second throttle valve to be returned to a predetermined opening angle since the second throttle valve has already returned to the fully open position at the time of the termination of the previous engine output control (as denoted by a dot-dot-and-dash line in FIG. 6), therefore responsiveness in controlling drive wheel slippage is reduced.
It is noted that such a response delay easily occurs under such conditions as, for example, when the vehicle runs with the first throttle valve fully opened or during a vehicle run on a low friction surface.
Furthermore, in a case where, in order to eliminate such a response delay as described above, the return speed of the second throttle valve toward the fully open position is made slower and a target opening angle of the second throttle valve toward which the second throttle valve is returned is set at the opening angle of the first throttle valve, in this case, an abrupt depression of an accelerator pedal by the driver with intention to abruptly accelerate after the end of the engine output control, (as denoted by a dot-and-dash line of FIG. 7) the opening angle of the second throttle valve is made smaller than that of the first throttle valve. Therefore, engine output is dependent on the opening angle of the second throttle valve and no rise in the engine output characteristic corresponding to the operation of accelerator pedal can be achieved and acceleration performance is degraded.